What's the difference between these two Blitter chips? How fast could they go? Atari Explorer has been quoted as saying the Atari Blitter could improve graphics and memory movement by as much as 16 times faster. Degas Elite draws about 7 times faster on my Mega ST4. Start Magazine has been quoated as saying the 8 MHz 68000 could outperform the Blitter in many operations refering to TurboST and QuickST for reference.

ROWBEARTOE wrote:What's the difference between these two Blitter chips? How fast could they go? Atari Explorer has been quoted as saying the Atari Blitter could improve graphics and memory movement by as much as 16 times faster. Degas Elite draws about 7 times faster on my Mega ST4. Start Magazine has been quoated as saying the 8 MHz 68000 could outperform the Blitter in many operations refering to TurboST and QuickST for reference.

I'm writing about the ST and Amiga and the more I know the better.

Thank you.

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Feature wise the OCS Amiga has

1) hardware line draw and area fill2) 3 source 1 destination blitter3) max size pre ecs 1024*1024 1 plane blit (realistically it maxes out at 1008*1024 depending on the alignment/shift) This is also per plane so it's pretty big3) concurrent operation with the cpu 1 out of every 4 cycles given to the CPU when in blit nice mode4) can be driven by the copper for beam sync blits5) with true fast RAM can run concurrently with the CPU6) 256 ways to combine sources and destination7) 0-15 bit shifts for source A and B

With a 32 colour screen and only CHIP ram1) 13% Slow down of the CPU2) 40% Slow down of the blitter 70824 bytes can be transferred at 100% theoretical blitter capacity 42602 actual blitter bytes can be transferred

with 4 planes the Amiga is about 33% fasterwith 5 planes the Amiga is slightly faster

Conclusion----------

Although cycle for cycle the Amiga blitter is 2x (copy), 3x (cookie cut) more efficient than the ST blitter DMA contention seriously reduces this advantage for 4 and 5 plane PAL screen modes.With a 5 plane PAL screen the graphical throughput of both machines is about even for drawingbobs. Ie the extra plane to process + DMA contention removes the efficiency advantage.The Amiga is slightly faster but not by a significant margin. Note the Amiga CPU speed in a 5 planePAL mode is also around 20% slower than the ST*

* I realise that fast RAM removes this disadvantage. A500s have never been shipped with true fast RAM, only chip and bogo fast.** Setup time can be drastically improved on the Amiga with interleaved bitmaps. It's possible to set the screen up so that only one blit is needed for the entire operation. On an A500 however this would mean using four or 5 times as much memory for the graphical mask. It would need to be n planes deep. The advantage would be concurrent operation with fast RAM and reduced set up time. The disadvantage would be the amount of chip memory it would take. It's trivial on the STE to have 4 meg of RAM which is effectively all CHIP memory. The Amiga maxes out at 2 meg of chip.

I'm also aware that you can get away with hog mode on the Amiga since all display list changescan be done with the copper which can always pre empt the Blitter.

The amiga blitter can tell you when a bit was set in the destination which means you can use it for collision detection.If you switch off the d channel it will not affect the dest but still tell you if the two regions over lap. Rather nice.The designers of both blitters knew what they were doing

That document I wrote was actually about ECS. Yes you're correct the original Agnus could only address 512k of chip memory.I think most A500s shipped with fat Agnus though. To answer your question the ST blitter is only really faster at clearing and filling memory. The Amiga is 2x more efficient at copying and 3 x more efficient at masking sprites. The point I was making in my previous post (and Cyprian K did too is that this efficiency is only achievable if the Amiga isn't displaying any graphics at all. Every bitplane you switch on costs you blitter cycles on the Amiga. By the time you have 5 planes in PAL mode switched on the difference is negligible for copying and about 50% on masking. If you're blitting 5 planes on the Amiga and 4 on the ST the difference isn't that great. I think you really need to be comparing a 68k cpu vs the blitter...

Actually it can't fill polygons directly. It would require intermediate buffer for that. Amiga could fill the screen in simple-flood-fill fashion and that would only enable convex shapes to be filled. There's even a special line drawing mode as this kind of fill required that no line would have more then one pixel on the same Y value. This kind of fill can be also done with ST blitter.

Amiga Bliter in OCS could access 1MB with Fat Agnus or 512KB with Agnus.Ok, thanks- now I'm going to have to read and try and understand what you have all posted. And somehow translate it into english should this be mentioned in a more detailed way with my comparison.

Thanks. The more information the better.To bad there was no simple benchmark-